Polychloroprene gasket-forming compositions containing as a rheologyaffecting additive, 2-mercaptobenzothiazole

ABSTRACT

POLYCHLOROPRRENE COMPOUNDS OF HIGH RUBBER CONCENTRATION, BOTH ON A WET AND DRY BASIS, CAN BE SUCCESSFULLY LINED AS GASKETS INTO AEROSOL MOUNTING CUPS IF THEY CONTAIN BETWEEN ABOUT 1.25 AND 4 PARTS BY WEIGHT PER HUNDRED PARTS RUBBER OF 2-MERCAPTOBENZOTHIAZOLE.

United States Patent 01 hoe 3,748,297 Patented July 24, 1973 US. Cl. 260--31.8 DR 3 Claims ABSTRACT OF THE DISCLOSURE Polychloroprene compounds of high rubber concentration, both on a wet and dry basis, can be successfully lined as gaskets into aerosol mounting cups if they contain between about 1.25 and 4 parts by weight per hundred parts rubber of Z-mercaptobenzothiazole.

THE PRIOR ART The use of aerosol containers for packaging and dispensing both fiuid and powdered particulate materials continues to grow considerably. Currently, a wide variety of products such as insecticides, paints, cosmetics, food products and pharmaceuticals are being marketed in this type of package.

A typical aerosol unit is composed of a hollow cylindrical container closed at one endand provided with an opening at its opposite end for receiving a dispensing valve assembly. A metal mounting cup serves as a 010- sure for the container and also as a support for the valve assembly which is tightly fitted within an aperture centrally located in the cup. The mounting cup carries an annular gasket adapted at its outer edge to be crimped to the walls forming the opening in the container and thereby form a seal upon crimping.

The gasketing material used in aerosol mounting cups must be elastic and resilient. It must be capable of yielding under deforming forces to establish a satisfactory seal when the cup is crimped to the container. It must also be capable of maintaining the seal during normal storage periods. Often selected for this function are gaskets prepared from solvent-based rubber compositions which comprise an elastomer dissolved in a volatile organic solvent. The compositions may also contain pigments, fillers, curing agents and other conventional compounding ingredients. Because of superior resistance to oxidation and to oils and propellants as well as to various other chemicals commonly found in aerosol containers, neoprene (a chlorobutadiene polymer) has become the favored elastomer in this type of composition.

To prepare gaskets from an elastomeric composition, a band of the fluid composition is deposited in the annular channel of the mounting cup while the cup is being rotated beneath a metering nozzle through which the composition flows to form a continuous annular deposit. This deposit is subsequently converted into a dry solid elastomeric sealing mass by expulsion of the solvent and cured at elevated temperatures.

It has been found, however, in the course of employing areosol mounting cup compounds of high chloroprene content, that air is frequently entrapped in the compound as it is applied to the annular channel of aerosol cups. This entrapment occurs as a massive air bubble in the lap area of the gasket which cannot break out during the curing process and which ultimately renders the chloroprene gasket ineffective as a sealing member of the assembly. The lap area is, of course, the location at which the deposition of the elastomeric compound into the annular channel of the cup is completed, there being created at that point a slight overlap of last deposited composition over first deposited composition.

SUMMARY OF THE INVENTION It has now been discovered that the inclusion of about 1.25 to 4 parts by weight of 2-mercaptobenzothiazole per hundred parts of rubber (phr.) into highly concentrated compositions of polychloroprene will permit rapid lining of aerosol mounting cups and yield, after cure, gaskets having a significantly reduced incidence of defects caused by the undesirable formation of air bubbles in the lap area of the gaskets. The mercaptobenzothiazole favorably affects the rheology of the rubber lining compound to such a degree that the reduced defect rate renders the compound usable on high speed lining equipment. In the absence of the thiazole at the level required, rubber compounds of this type are so sensitive to mechanical adjustment of the high speed application machinery that they are not considered commercially useful in spite of their otherwise excellent properties.

DETAILED DESCRIPTION OF THE INVENTION The beneficial function of Z-mercaptobenzothiazole just described is rather surprising in view of its conventional role in rubber preparations. The thiazole is generally recommended as a cure accelerator for natural rubber, and ethylene-propylene terpolymer. With chloroprene rubbeen said to act as a mild retarder, probably in the manner of its oxidation product, dibenzothiazyl disulfide, when used at levels of 0.5 to 1.0 phr. (The Vanderbilt Rubber Handbook, R. T. Vanderbilt Company, Inc., New York 1968; pages 252-3).

The compositions that can benefit from the present invention consist essentially of a polymer of 2-chloro-1,3- butadiene dissolved in a volatile organic solvent. The elastomeric polymer may be either a homopolymer of the chloroprene or a copolymer containing minor quantities of styrene, acrylonitrile, isoprene and other monomeric units copolymerizable with it. These chloroprene polymers are commonly referred to as neoprenes and are commerically available as general-purpose types, such as GN, GNA, GRT, W, WRT and WHV. Any general purpose neoprene may be used alone or in admixture. Also, special purpose neoprenes, such as KNR, CG and AC, may be used in combination with general-purpose types.

The chloroprene polymer may be dissolved in a single volatile organic liquid or in mixture of such liquids able to function together as solvent medium. Preferred usable organic liquids in this capacity are those that have a minimum boiling point of 230 F., as well as mixtures of liquids that do not boil below 230 F. and yet are capable of volatilization in the course of the overall drying and curing cycle to which the gasketing compounds are subjected. Illustrative of this type of liquids are toluene, xylene, a high boiling petroleum solvent such as Solvesso wtih a boiling range of 230 F. to 345 R, an aliphatic petroleum solvent such as Varsol #1 which boils between 320 F. and 390 F., and mixtures of such liquids.

The volatile Organic liquid selected is used in quantities such that compositions having a total solids concentration between about 40 and 50% by weight, and preferably between about 45 and 50% by weight, are obtained. The total solids portion of the compositions includes the combined weight of polymer, fillers, curing mixture and the other substantially non-volatile ingredients employed. The amount of the polymer in the composition should be at least about 21% of the weight of the total composition.

The curing agent may be a metallic oxide such as magnesium oxide, zinc oxide, a lead oxide, or mixtures of elemental sulfur with a metallic oxide. About 1 to 15 phr. of curing agent is generally used, any excess over that level acting as filler. Preferably, a mixture of oxides is used that contains about 4 phr. of a light or extra light calcined magnesia per 5 phr. of zinc oxide, such a mixture providing relatively rapid and complete cure with minimum scorching. A curing accelerator may be used in conjunction with the oxides, e.g. Z-mercaptoimidazoline or p,p'-diaminodiphenylmethane.

The compositions may include a non-volatile liquid plasticizer in quantities ranging between about 0 and 20 phr. Though not essential, plasticizers facilitate mixing of the polymer with other ingredients such as fillers and also enhance the sealing characteristics of the resulting gasket. Preferably, the plasticizer is used at a level not greater than phr. Illustrative plasticizers include dioctyl sebacate, dioctyl adipate, didecyl phthalate, dioctyl phthalate, naphthenic oils or any relatively non-volatile liquid plasticizing material conventionally used as processing aid for neoprene rubber.

While the presence of inorganic fillers is not essential in the present compositions, their use in sealing aerosol containers tends to reduce the permeability of the gasket to propellants. Fillers are also helpful in modifying the specific gravity and flow characteristics of the fluid com position. Suitable fillers include hydrated calcium silicate, fine sized Whiting, talc, silicon dioxide and clays. The quantity of filler may range between about 0 and 100 phr. Quantities ranging between about 30 and 70 phr. are preferred, the exact quantity used depending upon the filler or combination of fillers selected, the characteristics imparted to the fluid composition, and the properties desired in the cured gasket.

It is also desirable to use a peptizer for the polymer. Peptizers allow greater ease in adjusting the viscosity of the composition to a value within the range required for standard lining equipment. Typical eptizers include alkyl thiuram disulfides, e.g. tetramethyl and tetraethyl thiuram disul fides; piperidinium alkyl dithiocarbamates, e.g. piperidiniumpentamethylene dithiocarbamate; and guanidines. Peptizers are used in quantities preferably between about 0.5 and 6 phr.

Other ingredients may be incorporated into the composition to impart desirable properties. These include lubricants, e.g. stearic acid and petroleum waxes; antioxidants, e.g. diphenyl-p-phenylenediamine and p-(p-tolylsulfonylamide)diphenylamine; and pigments, e.g. carbon black, iron oxide and titanium dioxide. When lubricants, antioxidants, pigments and other ingredients are employed, they are added in conventional quantities to achieve the desired effect.

The following examples are given to further illustrate the invention. All quantities mentioned are in parts of weight unless otherwise specified.

Highly concentrated polychloroprene compounds were prepared with difierent types and levels of fillers and varying cure accelerators. The compounds were applied to aerosol mounting cups and cured in the conventional manner. The gaskets obtained were inspected for evidence of lap bubble formation, a phenomenon which ultimately leaves a hole in the gasket as the air entrapped during the compound deposition escapes after the compound has begun to cure. Two of these high concentration polychloroprene compounds are described in the following table (Examples 1 tnd 2), one of which contains 2-mercaptobenzothiazole (Example 2).

On visual examination of gaskets made from these compounds, it was observed that those obtained from the composition of Example 1 were very poor, while those yielded by the thiazole-containing compound were rated excellent to fair. The most remarkable effect of the thiazole on the behavior of the high chloroprene compounds was the favorable modification of the rheology of these preparations so that they could be handled successfully by high speed commercial equipment. No such problem has been found to arise in the lining of conventional chloroprene compositions, such as those described in US. Pat. 3,389,113, in which the higher proportions of plasticizer and filler and the resulting lower chloroprene content facilitate the :fiow of the compound through the nozzle of the lining machinery onto the mounting cups.

Other high concentration polychloroprene compounds were prepared to corroborate the data previously obtained. Their composition was the following:

Example- 3 4 5 6 Fixed Ingredients (parts by weight):

Polychloroprene (Neoprene GN)- 100 100 100 Paraflin wax 2.0 2.0 2.0 2. 0 Sodium acetate 1. 5 1. 5 1. 5 1. 5 Magnesium oxide 8. 0 8. 0 8. 0 8. 0 Tetraethylthiuram disulfide- 2. 5 2. 5 2. 5 2. 6 Piperidinium-pentamethylene dithiocarbarnat-e 2. 0 2. O 2. 0 2. 0 Zinc oxide 5. 0 5. 0 5. 0 5. 0 Dioetyl 7. 4 7. 4 7. 4 7. 4 Silica ge 0.1 0. 1 0. 1 0.1 Ziuce resinate O. 3 0. 3 0. 3 0. 3 Xylene 200 200 200 200 Variable ingredients (parts by weight):

2-mercaptobenzothiazole 2 2 Aluminum silicate (kaolin) 50 60 Calcium carbonate I 50 50 A Darex #9 ACF automatic compound lining machine for aerosol mounting cups was used in these tests. The machine was adjusted so that the compound of Example 5 was lined with lap bubbles. Then, without disturbing the lap setting of the machine, the compounds of Example 3, 6 and 4 were lined in that order. Approximately 300 cups were lined with each of the four compounds. The only variance used in lining each compound was the pressure used to regulate the quantity of material. This is necessary to achieve a substantially constant quantity of cured compound among all compounds. Enough of each compound was thus applied to give gaskets of 0.023 inch in thickness having the following film Weights: 605 mg., 625 mg., 625 mg. and 630 mg. for the compounds of Examples, 3, 4, 5 and 6 respectively. After lining, the compounds were put through a normal aerosol gasket curing cycle of 1 hour air drying, 1 hour heating at F., 1 hour at 250 F. and 1 hour at 325 F.

From each set of lined and cured cups, 25 cups were randomly selected. These cups were marked and placed into a bag. The four sets of 25 cups were then visually rated by three persons using a chart of standard defects for comparison purposes. The defects were scored on a scale of 1 to 6 with 1 being scored by the best cups. The average of 3 observations per cup, and 75 observations per set, were assessed statistically by an analysis of variance technique (ANOVA) to establish whether the observed effects were real or due to experimental error. The results were as follows:

point of 230 F., to yield a non-volatile component concentration within the range of 40 to 50% by weight, said It can correctly be concluded from these data, which demonstrate that the efifects are real, that more defects are produced when kaolin is used as the filler and that the inclusion of 2-mercaptobenzothiazole in the compounds significantly reduces the occurrence of lap bubbles. This, as intimated earlier, is due to a modification of the rheological properties of the high polychloroprene compounds which ultimately renders such compounds available for use with high speed gasket laying machinery.

It is apparent from the above description that it is possible to vary the nature and level of various components of high polychloroprene compositions Without departing from the spirit of the invention, the scope of which is limited only by the following claims.

What is claimed is:

1. A composition for forming flowed-in gaskets comprising, on a weight basis, 100 parts of a polymer of 2- chloro l,2-butadiene, between 0.5 and 6 parts of a peptizer, between 1 and 15 parts of a curing agent, between 0 and 20 parts of a non-volatile plasticizer for the polymer, between 0 and 100 parts of a filler, between 1.25 and 4 parts of Z-mercaptobenzothiazole, and sufficient volatile liquid hydrocarbon solvent, having a minimum boiling References Cited UNITED STATES PATENTS 3,389,113 6/1968 Simons et a1 26041.5 R 3,310,524 3/1967 Hurlock et a1. 26045.8 SN 3,220,968 11/1965 Dollhausen et a1. 2603 1.8 DR 2,802,811 8/1957 Somerville et a1. 26045.8 SN

MORRIS LIEBMAN, Primary Examiner P. N. THOMAS, JR., Assistant Examiner U.S. Cl. X.R.

26031.8 H, 33.6 A, 41.5 R, 41.5 A 

